D.C. Generators

Introduction

Although a far greater percentage of the electrical machines in service are a.c.
machines, the d.c. machines are of considerable industrial importance. The
principal advantage of the d.c. machine, particularly the d.c. motor, is that it
provides a fine control of speed. Such an advantage is not claimed by any a.c.
motor. However, d.c. generators are not as common as they used to be, because
direct current, when required, is mainly obtained from an a.c. supply by the use
of rectifiers. Nevertheless, an understanding of d.c. generator is important
because it represents a logical introduction to the behaviour of d.c. motors.
Indeed many d.c. motors in industry actually operate as d.c. generators for a
brief period. In this chapter, we shall deal with various aspects of d.c.
generators.

1.1 Generator Principle

An electric generator is a machine that converts mechanical energy into
electrical energy. An electric generator is based on the principle that whenever
flux is cut by a conductor, an e.m.f. is induced which will cause a current to flow
if the conductor circuit is closed. The direction of induced e.m.f. (and hence
current) is given by Fleming’s right hand rule. Therefore, the essential
components of a generator are:
(a) a magnetic field
(b) conductor or a group of conductors
(c) motion of conductor w.r.t. magnetic field.
1.2 Simple Loop Generator
Consider a single turn loop ABCD rotating clockwise in a uniform magnetic
field with a constant speed as shown in Fig.(1.1). As the loop rotates, the flux
linking the coil sides AB and CD changes continuously. Hence the e.m.f.
induced in these coil sides also changes but the e.m.f. induced in one coil side
adds to that induced in the other.
(i) When the loop is in position no. 1 [See Fig. 1.1], the generated e.m.f. is
zero because the coil sides (AB and CD) are cutting no flux but are
moving parallel to it
(ii) When the loop is in position no. 2, the coil sides are moving at an angle
to the flux and, therefore, a low e.m.f. is generated as indicated by point
2 in Fig. (1.2).
(iii) When the loop is in position no. 3, the coil sides (AB and CD) are at
right angle to the flux and are, therefore, cutting the flux at a maximum
rate. Hence at this instant, the generated e.m.f. is maximum as indicated
by point 3 in Fig. (1.2).
(iv) At position 4, the generated e.m.f. is less because the coil sides are
cutting the flux at an angle.
(v) At position 5, no magnetic lines are cut and hence induced e.m.f. is zero
as indicated by point 5 in Fig. (1.2).
(vi) At position 6, the coil sides move under a pole of opposite polarity and
hence the direction of generated e.m.f. is reversed. The maximum e.m.f.
in this direction (i.e., reverse direction, See Fig. 1.2) will be when the
loop is at position 7 and zero when at position 1. This cycle repeats with
each revolution of the coil.

Note that e.m.f. generated in the loop is alternating one. It is because any coil
side, say AB has e.m.f. in one direction when under the influence of N-pole and
in the other direction when under the influence of S-pole. If a load is connected
across the ends of the loop, then alternating current will flow through the load.
The alternating voltage generated in the loop can be converted into direct
voltage by a device called commutator. We then have the d.c. generator. In fact,
a commutator is a mechanical rectifier.